General background. The intensive efforts by numerous research teams over the last 60 years have resulted in numerous processes and inventions on how to convert methane into higher hydrocarbons or other industrially useful compounds. A series of authors previously succeeded in discovering and clearly identifying a range of efficient and economical chemical reaction schemes for converting methane into higher alkanes/alkenes. An overview of these different approaches to convert methane into higher value products are given in the recently published patent applications WO 2004/041399, US 2005/0171393 or by Degirmenci et al., 2005 and shall not be reproduced here. Unfortunately, no such proposed process is readily applied at a large scale since they either involve the use of expensive reagents or excessive amounts of energy, or they are very inefficient.
Halogen routes to methane conversion. First attempts to convert methyl chloride into ethylene or propylene and other products on zeolite catalysts date back to U.S. Pat. No. 4,769,504, Lersch and Bandermann, 1991, White, et al., 1992, Murry, et al., 1994, and Sun, Yao et al., 1993, who used ZSM-5 type oxide based catalysts. U.S. Pat. No. 5,001,293 and U.S. Pat. No. 5,087,786 used a copper aluminium borate to manufacture lower alkanes. Investigations on the Lewis acidity in zeolites have been published by Sokol et al., 2000.
Particularly in the last few years, halides have been suggested as intermediates in the conversion of methane to useful products. As a consequence, various patents and a series of publications were made for the halogenation of methane to methyl chloride, methyl fluoride, methyl bromide or other methane derivatives containing halogen atoms as disclosed in U.S. Pat. No. 3,979,470, U.S. Pat. No. 4,523,040, U.S. Pat. No. 4,804,797, U.S. Pat. No. 6,452,058 and WO 2005/104689. These patent applications and patents were all motivated by the discoveries of Olah et al., 1973 in U.S. Pat. No. 4,523,040 who found excellent selectivities to mono-halogenated methanes in the eighties during the Lewis acid assisted chlorination or bromination of hydrocarbons, especially methane.
Other approaches involve the use of a metal halogenide as an oxidant for methane. The published US patent application no. 2004/0006246 A1 discloses a process wherein a reactant comprising an alkane, an alkene or an aromatic reacts with a metal halide to produce the halide of the reactant and reduced metal. The reduced metal is oxidized with air or oxygen to form the corresponding metal oxide. This metal oxide is then reacted with the halide of the reactant to form the alcohol and/or the ether corresponding to the original alkane, alkene or aromatic and the original metal halide, which is recycled. This process offers a readily accessible source of methylbromide, amongst others. The later document, WO2005/019143 A1, further claims processes for synthesizing olefins, alcohols, ethers and aldehydes, which involve the use of solid phase catalysts/reactants in addition to the metal oxides and metal halides. In addition it comprises techniques for improving the selectivity of these reactions.
A modification of the above process is described in a series of patents and patent applications: U.S. Pat. No. 6,462,243, EP 1 435 349 A2, U.S. Pat. No. 6,472,572 B1 and U.S. Pat. No. 6,486,369 B1 that disclose another process for manufacturing alcohols, ethers and olefins from alkanes. In a first step an alkane and bromine are mixed in a reactor to form alkyl bromide and hydrogen bromide. The alkyl bromide and the hydrogen bromide are directed into contact with a metal oxide to form an alcohol and/or an ether or an olefin and metal bromide. The metal bromide is oxidized to form original metal oxide and bromine. Both are recycled.
Similarly, WO 02/094751 discloses an oxidative halogenation process involving a reactant hydrocarbon selected from methane, a halogenated C1 hydrocarbon, or a mixture thereof with a source of halogen and a source of oxygen in the presence of a rare earth halide or a rare earth oxyhalide catalyst, so as to form a halogenated C1 hydrocarbon. This product can be condensed catalytically as disclosed in U.S. Pat. No. 5,397,560 to from light olefins, such as ethylene, propylene, butenes, and higher hydrocarbons, including C5+ gasolines. As catalysts, various zeolithes of the ZSM structure code are mentioned. The reaction temperature is typically greater than 250° C. A very interesting approach is offered by the U.S. Pat. No. 4,973,776 by Allenger and Pandey: They describe the conversion of natural gas or methane and acetylene on solid superacids comprising a binary metal fluoride (e.g. TaF5—AlF3) to isobutene and subsequent conversion to gasoline-range hydrocarbons in the presence of a silicalite zeolite catalyst. Yilmaz et al., 2005 and Zhou, Xiao-Ping et al., 2003 and U.S. Pat. No. 6,472,572 B1 and U.S. Pat. No. 6,486,368 B1 found, amongst others, a process for the conversion of ethane to ethylene, ethanol, diethyl ether and other products over oxide catalysts. Similarly, Lorkovic et al., 2004 (2 papers), Noronha et al., 2005 and Sun, Shouli et al., 2004, US 2004/0006246 A1, WO 86/04577 and U.S. Pat. No. 6,462,243 B1 describe bromine using, oxide mediated methane conversions.
WO 2004/041399 A2 describes an anhydrous sulfur based approach to methane activation. WO 2004/041399 details processes for the conversion of methane to methanol and lists anhydrous conditions as advantageous but only deals with oxygen containing products and intermediates.
Now, in order to better understand the background of the invention, it is useful to compare the out-lined existing processes and gather their characteristics: All processes involving halogens as reagents or adjuvants during the conversion of methane into useful products are confronted with the problem of converting the such generated halogenated methane into useful products. All processes for the conversion of methane inherently require the presence of carbon, hydrogen and at least one halogen, most frequently chlorine or bromine derivatives. Furthermore, all processes involve the formation of the corresponding H-Halide compounds at one step of the process. Such HCl or HBr are then generally recycled through oxidation, optionally via a metal salt intermediate. Catalysts or intermediate salts in all here described processes contain oxygen or oxides at least in one part of the process. The specific problems occurring due to the presence of H-Hal compounds and oxygen containing compounds including water, oxides, and especially zeolites, e.g. ZSM-5 like zeolites, are in more detail addressed below.
A careful analysis of why numerous processes of high attractiveness have failed commercialization in the past, reveals the dominant role of several factors that are often neglected when developing processes at a small scale:                1. Reactor stability. Attractive reactor or reaction design must enable a corrosion reduced environment to maintain operation over an extended period of time. Corrosion may come from high temperature, rapid temperature changes, aggressive chemicals, and others.        2. Catalyst stability. The active materials involved in the preparation of large-scale commodity materials of very low price and margins must be stable or recyclable over hundreds or even thousands of cycles. Therefore, such systems must be fully recycled, or they are virtually made de novo during each cycle of the process.        3. Energy requirements. In order to get a reasonable part of the methane converted into useful products, the energy requirements per cycle must inherently be low and the conversion of methane per cycle should be as high as possible, best close to 100%.        
While the halogen assisted methane activation offers a most attractive route to useful products, the yet existing processes may be split in three parts:                A) Methane halogenation (formation of methyl chloride or bromide)        B) Conversion of methyl halogen derivative (e.g. conversion to C3 to C5 hydrocarbons or oxygenates)        C) Recycling of H-Halogen compounds to the halogen itself (e.g. chlorine or bromine)        
Part A has been repeatedly described in the open scientific literature (e.g. Olah et al., 1985) and part C is also implemented into current industrial processes for the manufacturing of bromine. Therefore, most of today possible innovation can be made in part B, or, more specifically, in the conversion to the desired products. This is also the step where the selectivity and conversion rate of different processes decide on the overall outcome and efficiency of a process. Hence, the present invention primarily deals with an improved process of part B.
Looking at the predominant three large-scale-production problems (1 to 3, above), the use of halogenated compounds inherently addresses the question of corrosion of catalysts and reactor materials. The skilled researcher will easily remember the chemical properties of hydrochloric or hydrobromic acid which involve highly aggressive attack of most construction materials, be it metals or oxides. The problem of handling large volumes of HCl, HBr and other halogen-hydrogen compounds is well known. Indeed, corrosion in the processes disclosed by some of the previously cited patents and publications do require sophisticated alloys even when processing small laboratory scale amounts. Other corrosion processes by using HCl and HBr are well known and attack even highly alloyed steels. Furthermore, the use of alumina silicates or other oxidic systems as catalysts suffers from equilibrium between the oxide (MOx) and the halogenide (Hal):MOx+yHHal=MO(x-2y)Haly+y/2H2O  (eqn. 1)
Unfortunately, most metal halides and oxyhalides are quite volatile. Therefore, catalysts under such conditions are suffering from a lack of long term stability. The shift of the equilibrium at lower temperatures inherently results in chemical transport of parts of the catalyst and deposition of catalyst material within other parts of the reactor. Finally, such deposits are clogging tubes and induce failure of valves or whole components while the loss of catalytic activity also affects the overall plant performance. This effect can easily be made visible e.g. by the following experiment:
A piece of quartz wool (100 mg) is placed in a quartz tube (inner diameter about 6 mm) and heated by an external furnace to temperatures in the range of 150 to 500° C. while passing a stream of methyl chloride (5 vol % in argon, 10 ml per minute) through the tube. After a few hours, solid white deposits are formed at the colder end of the tube where it leaves the hot region downstream of the quartz wool. This readily demonstrates the volatility of a normally quite resistant and inert material like quartz wool. The outlined experiment can also be made with other silica based materials and even ceramic oxides undergo partial chemical transport. Active metals are often described as suitable dopants on zeolite catalysts. But e.g. many metals described as suitable on ZSM-5 in PCT/US2005/012655, undergo chemical transport as described above. This illustrates readily and clearly that such oxidic materials are of limited applicability for the conversion of methyl chloride or methyl bromide. Still, numerous authors have claimed patents in this area and used such oxide based materials.
Several catalysts have been proposed in connection with hydrocarbon modification. U.S. Pat. No. 3,578,725 discloses an aluminum halide catalyst for isomerizing hydrocarbons and GB patent 1212446 proposes the use of a catalyst system comprising a trihalonickelate complex and a lewis acid for dimerization of olefins.
PCT/US2005/012655 also lists the advantageous use of water free feed streams during bromination of methane but only lists a suppressed formation of carbon dioxide as a motivation.
Problems arising from handling HBr are described in great detail by Smudde et al., 1995. This work clearly shows how mixtures of water vapor of as low as 1700 ppm and air-contaminated HBr are corrosive to even highly alloyed stainless steels even comprising Hastelloy C-22 or Ni-200 and EP 316L.